Characterization of Steric and Electronic Properties of NiN2S2

Abstract: The physical properties and structures of a series of six complexes of the type (NiN2S2)W(CO)4 have been used to establish electronic and st...
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Characterization of Steric and Electronic Properties of NiN2S2 Complexes as S-Donor Metallodithiolate Ligands Marilyn V. Rampersad, Stephen P. Jeffery, Melissa L. Golden, Jonghyuk Lee, Joseph H. Reibenspies, Donald J. Darensbourg, and Marcetta Y. Darensbourg* Contribution from the Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77843 Received July 26, 2005; E-mail: [email protected]

Abstract: The physical properties and structures of a series of six complexes of the type (NiN2S2)W(CO)4 have been used to establish electronic and steric parameters for square planar NiN2S2 complexes as bidentate, S-donor ligands. According to the ν(CO) stretching frequencies and associated computed CottonKraihanzel force constants of the tungsten carbonyl adducts, there is little difference in donor abilities of the five neutral NiN2S2 metallodithiolate ligands in the series. The dianionic Ni(ema)2- (ema ) N,N′ethylenebis(2-mercaptoacetamide)) complex transfers more electron density onto the W(CO)4 moiety. A ranking of donor abilities and a comparison with classical bidentate ligands is as follows: Ni(ema)) > {[NiN2S2]0} > bipy ≈ phen > Ph2PCH2CH2PPh2 > Ph2PCH2PPh2. Electrochemical data from cyclic voltammetry find that the reduction event in the (NiN2S2)W(CO)4 derivatives is shifted to more positive potentials by ca. 0.5 V compared to the ca. -2 V NiII/I redox event in the free NiN2S2 ligand, consistent with the electron drain from the nickel-dithiolate ligands by the W(CO)4 acceptor. Differences in NiII/I ∆E1/2 values appear to have a ligand dependence which is related to a structural feature of the hinge angle imposed by the (µ-SR)2 bridges. Thus the angle formed by the intersection of NiN2S2/WS2C2 planes has been established by X-ray diffraction analyses as a unique orientational feature of the nickel-dithiolate ligands in contrast to classical diphosphine or diimine ligands and ranges in value from 136 to 107°. Variabletemperature 13C NMR studies show that the spatial orientations of the ligands remained fixed with respect to the W(CO)4 moiety to temperatures of 100 °C.

Introduction

Scheme 1

The development of innovative ligands for specific catalytic applications is of continuous interest in both academia and industry. Indeed the “art” of homogeneous catalysis typically lies in the minor chemical modifications of steric or electronic properties made to the ligands, such as phosphines, amines, and imines, N-heterocyclic carbenes, cyclopentadienides, and so forth, that tune a metal’s ability to control differences in rates of catalysis and product specificity.1 The discovery of entirely new classes of ligands with unique steric demands or long-term stability toward oxidative damage motivates many synthetic efforts. Conceivably, a recent and dramatic discovery of a new ligand class is to be found in the natural assembly of a catalytic reactive center within acetyl-CoA synthase (ACS).2-4 This bimetallic enzyme active site (see Scheme 1) has been deconvoluted into a nickel-dithiolate moiety, that is, a square planar NiN2S2 (1) (a) Van Leeuwen, P. W. N. M. Homogeneous Catalysis, Understanding the Art; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2004. (b) Van Leeuwen, P. W. N. M.; Kamer, P. C. J.; Reek, J. N. H.; Dierkes, P. Chem. ReV. 2000, 100, 2741-2769. (2) Dokov, T. I.; Iverson, T. M.; Seravalli, J.; Ragsdale, S. W.; Drennan, C. L. Science 2002, 298, 567-572. (3) Darnault, C.; Volbeda, A.; Kim, E. J.; Legrand, P.; Verne`de, X.; Lindahl, P. A.; Fontecilla-Camps, J. C. Nat. Struct. Biol. 2003, 10, 271-279. (4) Svetlitchnyi, V.; Dobbek, H.; Meyer-Klaucke, W.; Meins, T.; Thiele, B.; Romer, P.; Huber, R.; Meyer, O. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 446-451. 10.1021/ja055051g CCC: $30.25 © 2005 American Chemical Society

derived from a Cys-Gly-Cys tripeptide sequence within the protein, which binds a second, catalytically active nickel through bridging cysteinate sulfur donors.2-7 The second nickel, des(5) Lindahl, P. A. Biochemistry 2002, 41, 2097-2105. (6) Seravalli, J.; Xiao, Y.; Gu, W.; Cramer, S. P.; Antholine, W. E.; Krymov, V.; Gerfen, G. J.; Ragsdale, S. W. Biochemistry 2004, 43, 3944-3955. J. AM. CHEM. SOC. 2005, 127, 17323-17334

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ARTICLES Scheme 2

ignated Nip in Scheme 1, mediates the assembly of the acetylCoA thioester from CH3+ (from the corronoid cobalt-methyl donor), carbon monoxide, and the elaborate thiolate, CoA.8,9 The role of the first nickel, Nid, in N2S2 coordination appears to be largely structural, that is, serving as a template for the tripeptide. The bidentate NiN2S2 ligand thus derived has the appropriate electronic (donor) features to support the C-C and C-S coupling chemistry of the second nickel, Nip. The chemistry performed by the ACS active site is related to well-known organometallic chemistry which takes place at 16-electron square planar complexes, including a current industrial process which has a remarkably similar catalytic motif involving nickel or palladium ligated with diphosphines, diimines, and mixed donor bidentate ligands. The CO/R migratory insertion that follows olefin binding to an acetyl-NiII or -PdII moiety is a key step in the homogeneous catalysis of CO/olefin copolymerization for production of polyketones.10-12 As shown in Scheme 1, a metal-mediated C-C coupling via migratory insertion is the function of both the biological and the industrial catalysts. That NiN2S2 complexes may be used as building blocks in the design of polymetallic complexes has been demonstrated for scores of compounds in numerous structural types.13-20 The important reaction chemistry observed in the ACS active site pointed to the potential of the NiN2S2 complex as a ligand in classic organometallic chemistry.3,8,18 Thus in preliminary studies, summarized in Scheme 2, we synthesized the (bme(7) Gencic, S.; Grahame, D. A. J. Biol. Chem. 2003, 278, 6101-6110. (8) Webster, C. E.; Darensbourg, M. Y.; Lindahl, P. A.; Hall, M. B. J. Am. Chem. Soc. 2004, 126, 3410-3411. (9) Amara, P.; Volbeda, A.; Fontecilla-Camps, J. C.; Field, M. J. J. Am. Chem. Soc. 2005, 127, 2776-2784. (10) Sen, A. Catalytic Synthesis of Alkene-Carbon Monoxide Copolymers and Cooligomers; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2003. (11) Shultz, C.; DeSimone, J.; Brookhart, M. J. Am. Chem. Soc. 2001, 123, 9172-9173. (12) Rix, F. C.; Brookhart, M.; White, P. S. J. Am. Chem. Soc. 1996, 118, 47464764. (13) Rao, P. V.; Bhaduri, S.; Jiang, J.; Holm, R. H. Inorg. Chem. 2004, 43, 5833-5849. (14) Jicha, D. C.; Busch, D. H. Inorg. Chem. 1962, 1, 872-877. (15) Golden, M. L.; Jeffery, S. P.; Miller, M. L.; Reibenspies, J. H.; Darensbourg, M. Y. Eur. J. Inorg. Chem. 2004, 231-236. (16) Miller, M. L.; Ibrahim, S. A.; Golden, M. L.; Darensbourg, M. Y. Inorg. Chem. 2003, 42, 2999-3007. (17) Jeffery, S. P.; Lee, J.; Darensbourg, M. Y. Chem. Commun. 2005, 9, 11221124. (18) Linck, R. C.; Spahn, C. W.; Rauchfuss, T. B.; Wilson, S. R. J. Am. Chem. Soc. 2003, 125, 8700-8701. (19) Hatlevik, O.; Blanksma, M. C.; Mathrubootham, V.; Arif, A. M.; Hegg, E. L. J. Biol. Inorg. Chem. 2004, 9, 238-246. (20) Harrop, T. C.; Olmstead, M. M.; Mascharak, P. K. Chem. Commun. 2004, 15, 1744-1745. 17324 J. AM. CHEM. SOC.

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daco)Ni complex derivative of Pd(CH3)2 as a stable precursor to the NiN2S2Pd(CH3)(solvent)+ complex which readily takes up CO. The rapid CO/CH3 migratory insertion process which followed is a prototype for the first and repeating steps in CO/ olefin copolymerization reactivity.21 The success of this test reaction implied that mixed metal complexes that are capable of catalytic activity might be similarly constructed from an available library of NiN2S2 ligands. Hence, characterization of the intrinsic properties of this class of ligands is of interest. Historically, the use of metal carbonyls as spectroscopic indicators of the electron-donating abilities of ancillary ligands has been highly successful in the development of homogeneous catalysts. Indeed, the vocabulary of organometallic chemistry that describes electronic and steric characteristics of ligands has its basis in the well-known Tolman parameters.22 While other approaches and definitions have been put forth over the years,23-25 those of Tolman, originally developed for monodentate phosphorus donor ligands in combination with the 16electron Ni0(CO)3 as acceptor, remain the simplest and most widely used. In fact, the validity of the use of CO stretching frequencies as measures of ligand basicity or donor ability has been challenged by results from photoelectron spectra of a series of (P-P)W(CO)4 complexes in which Me and Ph substituents on the P-donors of diphosphines were systematically varied.26,27 Nevertheless, as the ν(CO) frequency values continue to be a reference point for comparisons for classical ligands, such as diphosphines, diamines, or diimines, we have prepared W(CO)4 derivatives of bidentate NiN2S2 complexes and recorded their IR spectra in the ν(CO) region. X-ray crystallographic analyses detail the unique spatial characteristics of a series of NiN2S2 complexes as ligands to W(CO)4. Carbon-13 NMR spectroscopic studies and electrochemical characterizations are also reported for the series. Several examples of (NiN2S2)M(CO)x complexes have been reported.18,28-33 From the CO stretching frequencies of a 2,3pentanedionebis(β-mercaptoethylimino)nickel-Mo(CO)4 complex, Kang and co-workers concluded that the NiN2S2 metallodithiolate ligand was a better donor than phosphines and thioethers and similar to bipyridine.28 Our own work with various complexes of the NiN2S2 metallothiolate ligand, (bmedaco)Ni, Ni-1, including [Ni-1]Fe0(CO)4, [Ni-1]2FeII(CO)22+, and [Ni-1]Mo0(CO)4 complexes,30,31 concurred with the Kang et al. study and established that the nickel dithiolate was only slightly poorer than the free, anionic thiolate as an electron donor. In more recent efforts to synthesize model complexes (21) Rampersad, M. V.; Jeffery, S. P.; Reibenspies, J. H.; Ortiz, C. G.; Darensbourg, D. J.; Darensbourg, M. Y. Angew. Chem., Int. Ed. 2005, 44, 1217-1220. (22) Tolman, C. A. Chem. ReV. 1977, 77, 313-348. (23) Perrin, L.; Clot, E.; Eisenstein, O.; Loch, J.; Crabtree, R. H. Inorg. Chem. 2001, 40, 5806-5811. (24) (a) Koide, Y.; Bott, S. G.; Barron, A. R. Organometallics 1996, 15, 22132226. (b) Hirota, M.; Sakakibara, K.; Komatsuzaki, T.; Akai, I. Comput. Chem. 1991, 15, 241-248. (c) White, D.; Taverner, B. C.; Coville, N. J.; Wade, P. W. J. Organomet. Chem. 1995, 495, 41-51. (25) Casey, C. P.; Whiteker, G. T. Isr. J. Chem. 1990, 30, 299-304. (26) Bancroft, G. M.; Dignard-Bailey, L.; Puddephatt, R. J. Inorg. Chem. 1986, 25, 3675-3680. (27) Lichtenberger, D. L.; Jatcko, M. E. J. Coord. Chem. 1994, 32, 79-101. (28) Kang, D.-X.; Poor, M.; Blinn, E. L. Inorg. Chim. Acta 1990, 209-214. (29) Yoo, J.; Ko, J.; Park, S. Bull. Korean Chem. Soc. 1994, 15, 803-805. (30) Chojnacki, S. S.; Hsiao, Y.-M.; Darensbourg, M. Y.; Reibenspies, J. H. Inorg. Chem. 1993, 32, 3573-3576. (31) Lai, C.-H.; Reibenspies, J. H.; Darensbourg, M. Y. Angew. Chem., Int. Ed. Engl. 1996, 35, 2390-2393. (32) Reynolds, M. A.; Rauchfuss, T. B.; Wilson, S. R.Organometallics 2003, 22, 1619-1625. (33) Krishnan, R.; Riordan, C. G. J. Am. Chem. Soc. 2004, 126, 4484-4485.

NiN2S2 Complexes as S-Donor Metallodithiolate Ligands

for the [NiFe] hydrogenase enzyme active site, Reynolds et al. concluded that the NiN2S2 metallothiolate ligands in [Cp*Ru(NiN2S2)(CO)]+ are better electron donors to RuII than two monodentate PMe3 ligands in [Cp*Ru(PMe3)2(CO)]+ or the bidentate diphosphine, Ph2PCH2CH2PPh2, in [Cp*Ru(dppe)(CO)]+.32 Other studies by Rauchfuss and co-workers and by Riordan and co-workers established that dianionic NiN2S2 complexes containing carboxamido nitrogens within the N2S2 donor environment formed dinickel complexes in which the adjacent nickel was Ni0 in mimicry of a possible redox level in the ACS active site.18,33 While such isolated reports provide a guide for expectations and design of complexes based on the metallo-ligands, the serial approach found herein broadly establishes ligating properties, including both electronic and steric properties, so that they may be generally applicable. Our study also points to the hemilability of such NiN2S2 ligands, a characteristic computed to be of note in the mechanism of the ACS enzyme.8 Experimental Section General Methods and Materials. All solvents used were reagent grade and were purified according to published procedures under an N2 atmosphere.34 The NiN2S2 complexes,35-40 cis-W(CO)4(pip)2 (pip ) piperidine),41 and complexes 1-4 were synthesized according to previously published procedures.21 All other chemicals were purchased from Aldrich Chemical Co. and used as received. Typically, anaerobic techniques were required, and an argon-filled glovebox and standard Schlenk-line techniques (N2 atmosphere) were employed. Physical Measurements. Canadian Microanalytical Services, Ltd., Delta, British Columbia, Canada, performed elemental analyses. Vis/ UV spectra were recorded in DMF on a Hewlett-Packard HP8452A diode array spectrometer. Infrared spectra were recorded on a Mattson Galaxy Series 6021 FTIR spectrometer in CaF2 solution cells of 0.1 mm path length. Photolysis experiments were performed using a mercury arc vapor, 450W UV immersion lamp purchased from Ace Glass Co. 13C NMR analysis was carried out with an INOVA 400 mHz Varian spectrometer. Electrochemistry. Cyclic voltammograms were recorded on a BAS-100A electrochemical analyzer using platinum wire as counter electrode, and Ag/Ag+, prepared by anodizing a silver wire in a CH3CN solution of 0.01 M AgNO3/0.1 M n-Bu4NBF4, was the reference electrode. The glassy carbon disk (0.071 cm2) working electrode was polished with 15, 3, and 1 µm diamond pastes, successively, and then sonicated in ultrapure (Millipore) water for 10 min. The solutions were purged with argon for 5-10 min, and a blanket of argon was maintained over the solution during the electrochemical measurements. All experiments were performed in CH3CN solutions containing 0.1 M n-Bu4NBF4 analyte at room temperature. Since the oxidation peaks for samples were overlapped with the Cp2Fe/Cp2Fe+ redox wave, Cp*2Fe served as the internal reference. The measured potential difference between Cp2Fe/Cp2Fe+ and Cp*2Fe/Cp*2Fe+ was 505 mV. Thus, all potentials are reported relative to the normal hydrogen electrode (NHE) using Cp2Fe/Cp2Fe+ as standard (E1/2 ) 0.40 V vs NHE in CH3CN).42 (34) Gordon, A. J.; Ford, R. A. The Chemist’s Companion; Wiley and Sons: New York, 1972; pp 429-436. (35) Kruger, H. J.; Peng, G.; Holm, R. H. Inorg. Chem. 1991, 30, 734-742. (36) Darensbourg, M. Y.; Font, I.; Pala, M.; Reibenspies, J. H. J. Coord. Chem. 1994, 32, 39-49. (37) Mills, D. K.; Reibenspies, J. H.; Darensbourg, M. Y. Inorg. Chem. 1990, 29, 4364-4366. (38) Smee, J. J.; Miller, M. L.; Grapperhaus, C. A.; Reibenspies, J. H.; Darensbourg, M. Y. Inorg. Chem. 2001, 40, 3601-3605. (39) Colpas, G. J.; Kumar, M.; Day, R. O.; Maroney, M. J. Inorg. Chem. 1990, 29, 4779-4788. (40) Grapperhaus, C. A.; Mullins, C. S.; Kozlowski, P. M.; Mashuta, M. S. Inorg. Chem. 2004, 43, 2859-2866. (41) Darensbourg, D. J.; Kump, R. L. Inorg. Chem. 1978, 17, 2680-2682.

ARTICLES Syntheses of (NiN2S2)W(CO)4 and (NiN2S2)W(CO)5 Complexes: [N,N′-Bis-2-mercaptoethyl-N,N′-diazacycloheptane]nickel(II) tungsten tetracarbonyl, (Ni-1′)W(CO)4, (5). The W(CO)4(pip)2 (0.17 g, 0.36 mmol) in 20 mL of CH2Cl2 was heated to 40 °C for 10 min under a N2 atmosphere. To this was added dropwise a yellow brown slurry of Ni-1′ (0.10 g, 0.36 mmol) dissolved in 10 mL of CH2Cl2. The resulting red-brown solution was heated for an additional 10 min at 40 °C. The solution was stirred for a few hours at 22°C, during which a tan-brown precipitate formed. The liquid was reduced to about 10 mL under vacuum, and the tan-brown solid was washed twice with 25 mL of benzene to remove excess piperidine and twice with 25 mL of ether. The product was dried under vacuum to yield 0.13 g, 61% of (Ni-1′)W(CO)4. X-ray quality crystals were grown by diffusion of ether into a DMF solution of the product. Anal. Calcd (found) for C13H18N2Ni1O4S2W1: C, 27.3 (26.9); H, 3.17 (3.67); N, 4.89 (5.27). Vis/UV in DMF solution λmax (): 308 (6728), 408 (1169), 450 (985), 512 (494) nm. IR (DMF, cm-1): ν(CO) 1996(w), 1873(S), 1852(m), 1817(m). 13C NMR (DMF): δ 211.7, 208.4, 203.8 ppm. [N,N′-Bis-2-methylmercaptopropyl-N,N′-dimethylethylenediamine]nickel(II) tungsten tetracarbonyl, (Ni(bmmp-dmed))W(CO)4, (6). In an identical manner for 5 above, a purple solution of Ni(bmmpdmed) was added to W(CO)4(pip)2 (0.15 g, 0.31 mmol). Isolation yielded 0.10 g, 53% of (Ni(bmmp-dmed))W(CO)4. X-ray quality crystals were grown by vapor diffusion of hexane into a CH2Cl2 solution of the product. Anal. Calcd (found) for C16H26N2Ni1O4S2W1: C, 31.1 (32.0); H, 4.25 (4.75); N, 4.54 (5.01). Vis/UV in DMF solution: λmax () 312 (5725), 388 (609), 440 (565) nm. IR (DMF, cm-1): ν(CO) 1998(w), 1878(S), 1854(m), 1821(m). 13C NMR (DMF): δ 213.5, 211.7, 210.1, 202.8 ppm. [N,N′-Bis-2-mercaptoethyl-N,N′-diazacyclooctane]nickel(II) tungsten pentacarbonyl, (Ni-1)W(CO)5, (7). A sample of W(CO)6 (0.1 g, 0.28 mmol) in 40 mL of THF under N2 was photolyzed for 2 h to yield a yellow solution of the W(CO)5(THF) adduct (ν(CO) ) 1975, 1931, 1892 cm-1). Addition of a purple slurry of Ni-1 (0.08 g, 0.28 mmol) in 20 mL of THF resulted in a red-brown solution that was stirred overnight at 22 °C. The volume was reduced in vacuo to about 10 mL; addition of ca. 40 mL of hexane produced a brown solid, which was washed with hexane (2 × 40 mL) to remove excess W(CO)6. The solid was further purified by silica gel column chromatography with CH2Cl2 as the eluent to remove excess Ni-1. The solid was dried under vacuum to yield 0.12 g, 72% of (Ni-1)W(CO)5. Anal. Calcd (found) for C14H26N2Ni1O5S2W1: C, 29.3 (28.9); H, 3.28 (2.92); N, 4.56 (4.25). IR (THF, cm-1): ν(CO) 2062 (w), 1974 (m), 1922 (s), 1884 (m). IR (DMF, cm-1): ν(CO) 2062 (w), 1972 (w), 1922 (vs), 1874 (m). 13CO Enrichment. Carbon-13-enriched W(CO) was prepared by 6 addition of 13CO gas to a CH2Cl2 solution of [PPN][W(CO)5Cl].43,44 From tungsten hexacarbonyl, the isotopically labeled cis-W(CO)4-n(13CO)n(pip)2 was prepared by the reported method.41 Replacement of the piperidine with the NiN2S2 ligand yielded the (NiN2S2)W(CO)4-n(13CO)n complexes. Samples containing approximately 0.050.07 mmol of the [(NiN2S2)W(CO)4-n(13CO)n] complexes in 0.7 mL of DMF were prepared under Ar and used in the VT NMR studies. X-ray Crystal Structure Determinations. Crystal data and details for data collection and refinement are given in Table 1. The crystals were mounted on a glass fiber at room temperature for the experiment. X-ray data were obtained on a Bruker P4 diffractometer. The space groups were determined on the basis of systematic absences and intensity statistics.45 Structures were solved by direct methods. Anisotropic displacement parameters were determined for all non-hydrogen atoms. Programs used for data collection and cell refinement: Bruker (42) Gagne, R. R.; Koval, C. A.; Lisensky, G. C. Inorg. Chem. 1980, 19, 28542855. (43) Cotton, F. A.; Darensbourg, D. J.; Kolthammer, B. W. S. J. Am. Chem. Soc. 1981, 103, 398-405. (44) Darensbourg, D. J.; Gray, R. L. Inorg. Chem. 1984, 23, 2993-2996. (45) Sheldrick, G. SHELXTL-PLUS, revision 4.11V; SHELXTL-PLUS users manual; Siemens Analytical X-ray Inst. Inc., Madison, WI, 1990. J. AM. CHEM. SOC.

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ARTICLES Table 1. Crystallographic Data for Complexes 4-7

formula formula weight temperature (K) wavelength (Å) Z Dcalcd (mg/cm3) µ (mm-1) crystal system space group a (Å) b (Å) c (Å) β (°) volume (Å3) goodness-of-fit R1a, wR2b (%) [I > 2σ(I)] R1a, wR2b (%) all data a

4

5

6

7

C14H20N2NiO4S2W 587.00 110(2) 0.71073 4 2.135 7.573 orthorhombic Pnma 13.397(4) 12.386(4) 11.005(3) 90 1826.2(10) 1.144 0.0279, 0.0603 0.0271, 0.0598

C13H18N2NiO4S2W 572.97 110(2) 0.71073 4 2.247 8.162 orthorhombic Pnma 12.721(5) 12.151(5) 10.959(4) 90 1693.8(11) 1.164 0.0719, 0.1781 0.0800, 0.1842

C17H28Cl2N2NiO4S2W 701.99 110(2) 0.71073 8 1.940 5.987 orthorhombic Pbca 18.325(11) 12.748(7) 20.575(12) 90 4806.5(5) 0.992 0.0490, 0.1078 0.0701, 0.1151

C15H20N2NiO5S2W 615.05 110(2) 0.71073 4 2.067 7.007 monoclinic P21/n 7.2847(12) 13.392(2) 20.266(3) 92 1976.3(6) 0.929 0.0626, 0.1403 0.1004, 0.1558

R1 ) ∑||Fo|-|Fc||∑Fo. b wR2 ) [∑[w(Fo2-Fc2)2]/∑w(Fo2)2]1/2. Chart 1. NiN2S2 Ligand Series (abbreviations, ∠S-Ni-S angles, and designations for W(CO)4 derivatives)

Scheme 3

XSCANS; data reduction, SHELXTL; structure solution, SHELXS9746 (Sheldrick); structure refinement, SHELXL-9747 (Sheldrick), and molecular graphics and preparation of material for publication, SHELXTL-Plus, version 5.1 or later (Bruker).48

Results and Discussion

The NiN2S2 metallothiolate ligands selected for this study are comprised of a square planar nickel within a N2S2 donor set; variations in structures arise from several features. The diazacyclooctane derivatives Ni-1 and Ni-1* have S-Ni-S ligand angles that are ca. 90°. The Ni-1′, based on diazacycloheptane framework, and the open-chain ligand derivative Ni(bmmp-dmed) have greater S-Ni-S ligand angles of 95°, resulting from a pinched N-Ni-N angle in both. The Ni-1* and Ni(bmmp-dmed) compounds possess gem dimethyl groups on the carbon R to the sulfur donor that should provide differences in steric and electron-donating properties of the thiolate sulfur. The Ni(bme-Me2PDA) is a neutral ligand that has a S-Ni-S angle less than 90°. Finally, the dianionic Ni(ema)2- has the largest S-Ni-S ligand angle of 97° and, obviously, the chief capability for donation due to its charge. This last metallodithiolate ligand also bears the closest resemblance to the NiN2S2 moiety in the ACS active site. The NiN2S2W(CO)4 derivatives were synthesized according to the labile ligand or ligand displacement approach (Scheme 3);21 simple addition of the NiN2S2 complex to the W(CO)4(pip)2 produced the NiN2S2W(CO)4 products which were isolated as crystalline materials, typically in g60% yields (Chart 1). Due to poor solubility in preferred nonpolar solvents, the ν(CO) stretching frequencies for the cis-L2M(CO)4 complexes (46) Sheldrick, G. SHELXS-97 Program for Crystal Structure Solution; Universita¨t Go¨ttingen, Germany, 1997. (47) Sheldrick, G. SHELXL-97 Program for Crystal Structure Refinement; Universita¨t Go¨ttingen, Germany, 1997. (48) SHELXTL, version 5.1 or later; Bruker Analytical X-ray Systems: Madison, WI, 1998. 17326 J. AM. CHEM. SOC.

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were measured in DMF. Although this solvent broadens the absorptions, peak maxima selected at (3 or 4 wavenumbers produced little differences in the computed Cotton-Kraihanzel (C-K) force constants.49 The ν(CO) values and the respective C-K force constants are reported in Table 2; for comparison, other cis-L2M(CO)4 (L ) diphosphine, bipyridine; M ) Mo) were prepared and their ν(CO) stretching frequencies also recorded in DMF. Representative infrared spectra in the ν(CO) region (Figure 1) display the four absorption bands that are expected for cisL2M(CO)4 complexes of idealized C2V symmetry, assigned to the two A1, B1, and B2 vibrational modes. It should be noted that the C2V symmetry designation only holds for the immediate S2(CO)4 donor environment about W; the significant asymmetry deriving from the hinge of the NiN2S2 ligand reduces the symmetry to Cs. Assuming C2V symmetry, the CO stretching vibrational mode assignments are according to that of Cotton and Kraihanzel: A11 corresponds to the band with the highest stretching frequency; B1 has the greatest intensity with a (49) Cotton, F. A.; Kraihanzel, C. S. J. Am. Chem. Soc. 1962, 84, 4432-4438.

NiN2S2 Complexes as S-Donor Metallodithiolate Ligands

ARTICLES

Table 2. CO Stretching Frequencies (cm-1)a and Calculated Force Constants (mdyn/Å)b

compound

Ni(bmmp-dmed)W(CO)4 (Ni-1*)W(CO)4 (Ni-1′)W(CO)4 (Ni-1)W(CO)4 Ni(bme-Me2PDA)W(CO)4 [(Ni(ema))W(CO)4]2-(Et4N)2 (Ni-1*)Mo(CO)4 (Ni-1′)Mo(CO)4 (Ni-1)Mo(CO)4 (dppm)W(CO)4c (dppe)W(CO)4c (dmpm)W(CO)4c (pip)2W(CO)4c (bipy)W(CO)4c

6 1 5 4 3 2 8 9 10 11 12 13 14 15

ν(A11)

ν(B1)

ν(A12)

ν(B2)

k1

k2

ki

1998 1996 1996 1995 1993 1986 2002 2002 2002 2016 2015 2007 2000 2006

1878 1871 1873 1871 1876 1853 1885 1886 1884 1906 1900 1885 1863 1886

1854 1857 1852 1853 1843 1837 1863 1851 1861 1906 1900 1885 1852 1870

1821 1816 1817 1819 1826 1791 1821 1824 1824 1870 1870 1863 1809 1830

13.77 13.74 13.74 13.77 13.81 13.41 13.80 13.81 13.84 14.50 14.50 14.39 13.68 13.94

15.00 14.99 14.98 14.95 14.91 14.77 15.17 15.10 15.13 15.43 15.34 15.10 14.94 15.19

0.38 0.43 0.41 0.41 0.35 0.46 0.41 0.37 0.40 0.38 0.38 0.38 0.46 0.41

a IR spectra were recorded in DMF. b The convention of modes and the calculated force constants follow that of ref 49. c Abbreviations: dppm ) bis(diphenylphosphino)methane; dppe ) bis(diphenylphosphino)ethane; dmpm ) bis(dimethylphosphino)methane; pip ) piperidine; bipy ) bipyridine.

Figure 1. Comparison of ν(CO) infrared spectra of (a) W(CO)4(pip)2 and (b) (Ni-1*)W(CO)4 in DMF solution.

shoulder or tail corresponding to A12; and the band of lowest frequency corresponds to B2.49 In the alternate approach, based on Cs symmetry, the W(CO)4 unit would similarly exhibit four vibrational frequencies of symmetry 3A′ + A′′. In the absence of stereoselectively 13CO-labeled species, it is not possible to unequivocally calculate individual force constants for the two different axial CO ligands. Hence, in treating the W(CO)4 moiety as C2V, an average axial CO force constant, k2, was calculated. The differentiation of axial CO groups will be addressed in a discussion of the temperature-dependent 13C NMR data observed for these complexes (vide infra). Among the neutral NiN2S2 ligands studied, only minor differences in ν(CO) stretching frequencies of the NiN2S2W(CO)4 were observed. The calculated Cotton-Kraihanzel force constants found that the k2 values (stretching force constant of

CO ligands cis to the NiN2S2 sulfur donors) for all the complexes are greater than the k1 values (force constant of CO ligands trans to NiN2S2 sulfur donors), consistent with the trans influence exerted by the auxiliary better donor ligands (L) in the pseudo-octahedral complex. The range of k1 or k2 values is small for the neutral metallothiolate ligands and indicates little differences in donor abilities. As expected, the dianionic Ni(ema)2- derivative transfers the most electron density to the W(CO)4 moiety, resulting in k1 and k2 values of 13.41 and 14.77 mdyn/Å, respectively. These are the lowest values of any compounds listed in Table 2. The neutral NiN2S2 sulfur donors result in CO stretching frequencies and force constants of W(CO)4 units that are most like the dipiperidine-substituted analogues; they are much better donors than diphosphines. On the basis of electrochemical results (NiII/I reduction couples), the complex Ni-1* with gem dimethyl groups on the carbon R to the sulfur is more electron rich than is Ni-1.50 Nevertheless, using the ν(CO) IR data in Table 2 as a criterion for donor ability, the two metallodithiolate ligands are indistinguishable. A reasonable conclusion is that the steric hindrance at sulfur prevents optimal close contact of the S-donor site of Ni-1* to the tungsten. Such arguments have been made for sterically bulky phosphine ligands in which the spatial requirements of the phosphines impinge on the innate electron-donating ability of the P-donor site, hence obviating a separation of steric and electronic parameters.26 X-ray Crystallography. As determined by X-ray diffraction analysis, the molecular structures of complexes 4-6 are shown in Figure 2, adding to the analogous (NiN2S2)W(CO)4 complexes 1-3, which were previously reported.21 Selected metric data for complexes 1-6 are listed in Table 3 along with that of the free metallodithiolate ligands for comparison. The molecular structure of the monodentate (NiN2S2)W(CO)5 complex 7 is (50) Buonomo, R. M.; Font, I.; Maguire, M. J.; Reibenspies, J. H.; Tuntulani, C.; Darensbourg, M. Y. J. Am. Chem. Soc. 1995, 117, 963-973. J. AM. CHEM. SOC.

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Figure 2. Thermal ellipsoid plots (50% probability) of the molecular structures for complexes 4-6 with select atoms labeled and hydrogen atoms omitted.

given in Figure 5, and a selection of distances and angles are given in the figure caption. A full list of bond lengths, bond angles, and anisotropic displacement coefficients for complexes 4-7 are in the Supporting Information. A common characteristic of the (NiN2S2)W(CO)4 structures is that the square planar NiN2S2 complexes bind as bidentate, S-donor ligands to W0, largely maintaining the structural features of the parent NiN2S2 complex with only minor deviations in metric parameters. In the case of Ni(ema)2-, however, the substantially planar structure changes into one with a Td twist of 8.6° on binding to W(CO)4, while the Ni-1′ distorts even more from planar (Td twist of only 2.1°) to a Td twist of 11.1° in the (Ni-1′)W(CO)4 complex. Interestingly, the NiN2S2 parent (free) ligand of complex 6 has a Td twist of 14.4° and has only a minor distortion to 16.5° on binding to W(CO)4. A second notable feature of the (NiN2S2)W(CO)4 structures is that the NiN2S2 plane in each case connects into the S2W(CO)2 plane of the pseudo-octahedral tungsten complex with a hinge at the sulfurs. This hinge, a consequence of the directionality of the sulfur lone pairs, is the hallmark of a majority of thiolate-bridged dinuclear compounds typically referred to as butterfly complexes. Rotation of the structural representation used that focuses on the octahedral geometry of the S2W(CO)4 coordination sphere into a form that better displays the butterfly character of the complexes (Figure 3) illustrates the difference in the chemical environment of the two CO groups that are mutually cis to the S-donors and trans to each other. In fact, the CO that is under the Ni‚‚‚W vector is ideally situated to be in a semibridging position. That it is strictly terminal is evident by the Ni-C(1) distances in the series of six complexes, which range from 2.85 to 3.72 Å and are beyond bonding. The shorter of these actually are within the sum of Ni + C van der Waals radii (1.63 + 1.77 ) 3.33 Å), suggesting the possibility of at least some orbital overlap. While there is no indication from ν(CO) infrared data of a low frequency CO that might indicate bridging character, NMR spectroscopic studies described below find two distinct resonances for the mutually trans CO carbons. 17328 J. AM. CHEM. SOC.

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The Ni‚‚‚W distance range, 2.92-3.39 Å, is beyond bonding; individual values largely correlate with the dihedral angle (d.a.) made by the intersecting NiN2S2 and WC2S2 planes which cover a range of 107-136°. Inspection of the crystal packing diagrams (see Supporting Information) and metric analysis finds that, in general, the closest intermolecular nonbonding contacts, in the range of 3.08-3.2 Å, are related to carbonyl oxygen orientations toward each other between layers of the neutral complexes. In the [Et4N]2[Ni(ema)W(CO)4] salt, tetraethylammonium cations intersperse between rows comprised of pairings of interdigitated anions. We conclude that the values of the dihedral angles in the series of complexes are only influenced by intramolecular electronic and steric interactions. Analysis of the differences in dihedral plane values benefits from comparisons of pairs of complexes in the series of six compounds. While the complexes with gem dimethyl groups on the carbon R to sulfur are expected to have the greatest spatial requirement within the W(CO)4S2 coordination sphere, in fact complex 1 with d.a. ) 135.6° realizes this prediction and complex 6 does not (d.a. ) 114°). Inspection of the structures of complexes 1 and 6 finds a distinct difference in the orientation of the gem dimethyl groups in the two complexes (Figure 4). Whereas in complex 1, two of the CR-CH3 vectors from each of the R-carbons are parallel and pointed in the vicinity of the W-C(1) bond vector, the open chain framework of complex 6 allows the analogous methyl groups to be splayed and oriented outwardly from the Ni(µ-SR)2W unit, offering little steric hindrance toward the W(CO)4 unit. Thus in the solid state at least, the steric encumbrance of the diazacycle-containing complex 1 is much greater than that of the open-chain complex 6. Complexes 4 and 5 also have large dihedral angles compared to those of other members of the series. In both cases, the C-C unit that links S to N is eclipsed across the NiN2S2 plane. The pucker in the five-membered NiSC2N rings displaces the C that is R to S in each case toward the W-C(1) bond vector, imposing steric hindrance from the H atoms of the R-CH2 groups in 4 and 5 that is nearly as great as the methyl substituent of complex 1. Complexes 2 and 3 have the smallest dihedral angles, 107°. The nearly flat character of the carboxamido N to S linkers accounts for the small steric requirement of complex 2. The open-chain SNNS ligand of 3 finds the CH2-CH2 units connecting N to S eclipsed across the NiN2S2 plane; however, unlike complexes 4 and 5, both carbons R to S in the NiSC2N five-membered rings are oriented down and away from the W-C(1) region. Hence, there is no obvious steric hindrance from the hydrogen atoms on that carbon. There is no apparent correlation of dihedral or hinge angle with other metric data of the complexes such as the S-W-S bite angle or W-S distances. The former ranges from 69 to 75° deriving from S-Ni-S angles of 83 to 98°. In all members of this series, the S-Ni-S angle decreases (by 1 to 4°) upon binding of the NiN2S2 ligand to tungsten, forming the W(CO)4 derivatives (Table 3). Minor differences in metric data exist in the Ni-S distances in members of the series of Ni-W bimetallics as compared to the free NiN2S2 ligands (see Table 3). Taken individually, these differences would not be statistically significant. From the series, however, the consistent observation of a slight increase in Ni-S bond length upon complexation to W, along with a concomitant

135.9

dihedralb

88.8(1) 90.4(1)

2.152(1) 1.995(3)

Ni-1* ref 36

107

176.5(5) 90.8(5) 173.4(11) 179.3(11) 75.06(9) 94.42(11) 86.04(4)

2.928 2.954 1.978(13) 2.042(12) 1.939(12) 1.937(12) 2.616(3) 2.172(3) 1.845(8)

2

97.44(8) 85.6(2)

2.179(1) 1.857(3)

[Ni(ema)]2ref 35

106.8

170.0(4) 89.9(4) 172.9(9) 170.8(8) 68.75(9) 83.83(11) 98.3(4)

3.033 2.846 2.023(13) 2.057(12) 1.952(8) 1.952(8) 2.591(2) 2.190(2) 1.995(6)

3

85.37(4) 97.3(1)

2.175(1) 2.005(3)

Ni(bme-Me2PDA) ref 39

132.5

172.4(3) 90.7(3) 171.7(7) 175.5(8) 72.08(6) 87.76(8) 91.5(3)

3.35 3.561 2.023(15) 2.047(11) 1.957(6) 1.957(6) 2.5792(14) 2.1893(15) 1.974(5)

4

89.4(1) 89.3(4)

2.159(3) 1.979(7)

Ni-1 ref 37

Averaged bond distances and estimated standard deviations are given in parentheses. b Angle between best planes of NiN2S2 and S2W(CO)2.

165.5(5) 91.3(6) 171.5(13) 173.3(12) 70.92(12) 87.62(15) 92.2(5)

C(1)-W-C(2) C(3)-W-C(4) W-C(1)-O(1) W-C(2)-O(2) S(1)-W-S(2) S(1)-Ni-S(2) N(1)-Ni-N(2)

a

3.389 3.726 2.001(14) 2.037(12) 1.951(10) 1.951(10) 2.589(3) 2.170(3) 1.986(8)

Ni-W Ni-C(1) W-C(1) W-C(2) W-C(3) W-C(4) W-Saavg Ni-Saavg Ni-Naavg

1

127.5

172.6(10) 91(9) 174.7(11) 174.9(10) 75(8) 92(9) 83(7)

3.249 3.388 2.03(4) 2.03(8) 1.96(15) 1.96(15) 2.5730(18) 2.17(16) 1.93(14)

5

95.4(1) 82.5(2)

2.164(1) 1.940(4)

Ni-1′ ref 38

Table 3. Selected Bond Distances (Å) and Bond Angles (Deg) of Complexes 1-6 and Ball-and-Stick Representations with NiN2S2 Ligands Included for Comparison

114.4

165.5(3) 91.3(4) 173.8(8) 178.8(8) 71.16(6) 91.49(9) 89.8(3)

3.021 3.24 2.052(9) 2.040(9) 1.970(9) 1.944(9) 2.621(2) 2.155(2) 1.994(7)

6

95.16(4) 88.11(12)

2.153(10) 1.940(3)

Ni(bmmp-dmed) ref 40

NiN2S2 Complexes as S-Donor Metallodithiolate Ligands ARTICLES

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Figure 3. The butterfly core defined by the (µ-SR)2 bridge in complex 2.

Figure 6. 13C NMR spectrum of (Ni-1)W(CO)4 in DMF at 22 °C in the low-field CO region. Asterisks (/) indicate 183W satellites and, ( indicate 13C-13C coupling. Assignments refer to the structure shown in Table 4.

Figure 4. Ball-and-stick representations of alternate views for (a) (Ni-1*)W(CO)4 and (b) (Ni(bmmp-dmed))W(CO)4, complexes 1 and 6, with focus on the orientation of the gem dimethyl groups of the carbons R to the S-donor atomssthe origin of the steric difference in the two molecules as displayed in the NiN2S2/WS2C2 dihedral or hinge angles: 136° for the former and 114° for the latter.

Figure 5. Two views of the molecular structures of complex 7, (Ni-1)W(CO)5: (a) thermal ellipsoid plot (50% probability) with atom labels and hydrogen atoms omitted, and (b) ball-and-stick drawing. Selected averaged distances (Å) and angles (deg): Ni-S(1), 2.176(3); Ni-S(2), 2.164(3); W-S(1), 2.577(3); Ni-Navg, 1.984(8); Ni‚‚‚W, 3.894; S-Ni-S, 88.5(11); N-Ni-N, 89.5(3); W-C(1)-O(1), 178.3(9); W-C(2)-O(2), 177.5(12); W-C(4)-O(4), 176.5(11); W-C(5)-O(5), 172.6(11); Ni-S(1)-W, 109.8(11).

decrease in Ni-N bond length from that of the precursor NiN2S2, is convincing evidence of change, presumably a decrease, in Ni-S bond character in the heterobimetallics. A different result is observed for the Ni(ema)2- metallodithiolate ligand. The Ni-S as well as the Ni-N distances slightly decrease (or remain the same) on binding to W. The greater Ni(dπ)-S(pπ) antibonding interaction in the dianionic complex and its partial relief on binding to tungsten could account for enhanced bonding in the Ni-W complex. The W-S bond distance in the [Ni(ema)W(CO)4]2- complex 2 of 2.616 Å is, however, within the range of that of the neutral compounds, 2.58 to 2.62 Å. Crystals of complex 7 were obtained inadvertently during crystallization attempts for complex 4. Complex 7 was subsequently prepared by the labile ligand approach from the W(CO)5(THF) adduct. Views of its molecular structure are given in Figure 5. The square planar NiN2S2 complex is again largely unchanged from that of the parent free ligand (least-squares 17330 J. AM. CHEM. SOC.

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plane deviation of Ni, N, and S ) 0.0698 Å), and the thiolate pendant arms are in the eclipsed position. The monodentate binding of the Ni-1 unit to W produces a long Ni-W metal distance of 3.89 Å, resulting from a W-S-Ni angle of 109.8°. At 2.176 Å, the Ni-S(1) distance for the tungsten-bound thiolate of 7 is 0.01 Å larger than that of the unbound Ni-S(2), 2.164 Å. Within the Ni-1 unit of complex 7, both Ni-S bond distances are increased as compared to those for the free Ni-1 ligand (2.159 Å). Both are shorter than the Ni-Savg ) 2.189 Å observed in the (Ni-1)W(CO)4 complex. The W-Savg distance of (Ni-1)W(CO)4, 2.579 Å, is the same as that in (Ni-1)W(CO)5, 2.577 Å. Only one W-C-O linkage deviates significantly from linearity (172.6°), and it is that closest to the unbound thiolate. Nevertheless, the S(2)-C(5) and S(2)-O(5) distances of 3.485 and 3.331 Å, respectively, are beyond bonding interactions. 13CO NMR Spectroscopy. The 13C NMR spectra of complexes 1-5, recorded at room temperature (22 °C) in DMF solvent on both natural abundance and randomly enriched C-13 samples, display three signals in the M-CO region of the spectrum in approximately 2:1:1 intensity ratio; as an example, the spectrum of complex 4 is shown in Figure 6. Chemical shifts for all complexes are listed in Table 4. The most intense resonance is assigned to the equivalent CO groups mutually trans to the S-donors of the NiN2S2 ligands and mutually cis to each other, labeled a and a′ in Table 4. Note that only for the highly unsymmetrical complex 6 are both a and a′ distinct; in all others, a ) a′, consistent with the mirror plane in the solidstate structures of all except complex 6. The nonequivalence of the carbonyls labeled b and b′ is imposed by the orientation of the NiN2S2 ligand. The assignment of b and b′ to specific resonances is not straightforward. From the dependence of C-13 chemical shifts of metal-bound carbonyls on electron density displaced onto the carbons, that carbon responsible for the resonance nearest to the position of the a/a′ carbonyl carbon is the more electron rich. Whether this CO is closest in proximity to the NiN2S2 plane, or opposite, in closer proximity to the remaining lone pairs on S, is, at this time, not known. The CO carbon resonances in the carbon-13-enriched samples have satellites that derive from 183W-13C and 13C-13C coupling (see Figure 6 and Table 4). The coupling constants for the CO ligands trans to the thiolate donors, J(183W-13C)trans-COa, are on the order of 170 Hz; they are larger than the coupling constants for the nonequivalent CO ligands cis to the thiolate

NiN2S2 Complexes as S-Donor Metallodithiolate Ligands

ARTICLES

Table 4. 13C NMR Data, 22 °C in DMF Solution (except where noted) for the Carbonyl Carbons in [(NiN2S2)W(CO)4] Complexes with the CO Designations

CO (trans to S) ligand

complex

a

Ni-1*

1a

212.2 183W-13C (172.0) a

Ni(ema)2Ni(bme-Me2PDA) Ni-1

2b 3 4

216.7 213.4 213.3 183W-13C (173.4) a

Ni-1′ Ni(bmmp-dmed)

5a 6

a 183W-13C

and

13C-13C

CO (cis to S) a′

b

b’

211.7

212.0 183W-13C (129.7) b 13C -13C (15.1) b b′ 209.1 210.1 209.3 183W-13C (123.2) b 13C -13C (15.3) b b′ 208.4 210.1

202.8 183W-13C (127.7) b′ 13C -13C (15.1) b b′ 207.1 204.2 203.3 183W-13C (127.5) b′ 13C -13C (15.3) b b′ 203.8 202.8

211.7 213.5

coupling measured on

13C-enriched

compounds. b CD3CN.

Figure 7. Variable-temperature 13C NMR spectra of (Ni-1*)W(CO)4 in DMF in the low-field CO region. Asterisks (/) indicate 183W satellites and ( indicate 13C-13C coupling shown in the 22 °C spectrum. Assignments of b and b′ are unconfirmed.

donors: J(183W-13C)cis-COb ) 130 Hz and J(183W-13C)cis-COb’ ) 128 Hz, respectively.51 Variable-Temperature (VT) 13C NMR Studies of [(NiN2S2)W(CO)4-n(13CO)n]. To determine the extent of CO site exchange in the W(CO)4 unit, particularly whether the CO’s labeled b and b′ might undergo rapid interconversion of sites, the temperature dependence of C-13 NMR spectra of selected complexes in the (NiN2S2)W(CO)4 series was recorded on randomly 13CO enriched samples dissolved in DMF. Displayed in Figure 7 are the spectra of [(Ni-1*)W(CO)4-n(13CO)n] measured over the range of 22 to 90 °C. The three characteristic resonances described above for complex 1 are seen in the 22 °C spectrum. The more intense resonance, which is assigned to the equivalent CO ligands trans to the thiolate donors, a, retains a narrow peak width over the temperature range. The resonances assigned to the nonequivalent CO ligands, b and b′, are sharp at 22 °C, they broaden as the temperature is raised, and they coalesce into the baseline at 90 °C. At the high-temperature (51) Buchner, W.; Schenk, W. A. Inorg. Chem. 1984, 23, 132-137.

limit of our experiment, 120 °C, there is evidence within the significant noise of the background at higher temperatures of the appearance of the site-averaged b + b′ signal at 208.4 ppm. On cooling the sample, the resonances of b and b′ reappear. Hence, we conclude that the observed CO site equilibration is a reversible phenomenon and is limited in this temperature range to the CO groups that are trans to one another. A coalescence of resonances for carbonyls b and b′ as seen for complex 1 did not occur at 90 °C or even above 100 °C for the neutral (NiN2S2)W(CO)4 complexes 4 and 5. Likewise, the four resonances of complex 6 retained their integrity up to 110 °C; higher temperatures were not experimentally accessible. Analogous VT NMR studies of the dianionic complex 2 were compromised by the ring-opening of the Ni(ema)2- followed by a CO redistribution process that produced a pentacarbonyl species analogous to complex 7. Activation barriers for the (CO)b/(CO)b′ site permutations in complex 1 were estimated from the following equations: k ) 1/τ; τcoalesence ) ((x2)π∆ν)-1; and ∆Gq ) RT [ln(R/Nh) ln(k/T)]). For complex 1, the exchange rate constant was calculated at the coalescence temperature of 90 °C (363 K), and the error indicated for the activation barrier is from computations made for coalescence temperatures of (10 on either side of 363 K.

τ ) 2.45 × 10-4 s (at 363 K) k) 4.08 × 103 s-1 (at 363 K) ∆Gq ) 64.4 ( 2.0 kJ/mol The process which equilibrates the (CO)b and (CO)b′ sites in complex 1 is expected to be a mutual buckling of the NiN2S2 and W(CO)4 units at the sulfur hinges; alternatively, this may be described as a double inversion at the sulfurs. Notably, the molecular mobility is greatest for the complex of greatest steric encumbrance and largest hinge angle. J. AM. CHEM. SOC.

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Electrochemical Studies. Cyclic voltammograms (CVs) of the (NiN2S2)W(CO)4 complexes were recorded at room temperature in CH3CN solutions containing 0.1 M n-Bu4NBF4 as supporting electrolyte. For direct comparison, CVs of the NiN2S2 metallothiolate ligands were examined under identical conditions; these are overlaid with their respective W(CO)4 derivatives in Figure 8. The electrochemical data are summarized in Table 5. In general, all of the neutral (NiN2S2)W(CO)4 complexes undergo one reversible reduction and one quasi-reversible oxidation within the acetonitrile solvent window. As the (pip)2W(CO)4 complex shows no reductions, the reversible reductions, found in the range of -1.36 to -1.64 V for complexes 1, 3, and 6, are assigned as the NiII/I redox couple. The shift to more positive potentials by ca. 0.50 V, as compared to the ca. -2 V NiII/I redox event in the free NiN2S2 ligand, is compatible with formation of a Lewis base-Lewis acid adduct with the W(CO)4 acceptor which withdraws electron density from the metalloligand via the bridging thiolate sulfurs. In contrast, the oxidation event seen in the neutral NiN2S2 ligands in the 0.20-0.40 V range is barely changed in the (NiN2S2)W(CO)4 complexes; these were proposed to be sulfur-based in the NiN2S2 free ligands.50 A second oxidative process, proposed to be nickelbased, is observed for the Ni-1* and Ni(bmmp-dmed) free ligands, but is shifted out of the CH3CN solvent window in the (NiN2S2)W(CO)4 complexes 1 and 6. Thus, if indeed the more accessible oxidation process is sulfur-based, it would appear to be less dependent on the formation of the W(CO)4 complex than are the Ni-based redox events. It should be mentioned that there is an irreversible oxidation in the CV of the (pip)2W(CO)4 complex at ca. +0.5 V. Hence, the assignment of the oxidation events displayed in Figure 8 is not definite. Table 5. Half-Wave and Anodic Potentials for Reductions and Oxidations of NiN2S2 and [NiN2S2]W(CO)4 Complexesa E1/2 (V)

Epa (V)

compound

reduction

1st oxidation

Ni-1 Ni-1′ Ni-1* Ni(bmmp-dmed) Ni(bme-Me2PDA) [Ni(ema)](Et4N)2 (Ni-1)W(CO)4 (Ni-1′)W(CO)4 (Ni-1*)W(CO)4 Ni(bmmp-dmed)W(CO)4 Ni(bme-Me2PDA)W(CO)4 [Ni(ema)W(CO)4](Et4N)2

-2.02 -2.03 -2.11 -1.97 -2.00

0.17 0.21 0.30 0.38 0.34 -0.30b 0.25 0.30 0.36 0.35c 0.36 -0.12

-1.56 -1.51 -1.64 -1.42 -1.36 -2.34d

2nd oxidation

1.15 1.04

a All potentials scaled to NHE referenced to a Cp Fe/Cp Fe+ standard 2 2 (E1/2NHE ) 0.40 V; see Experimental Section). In CH3CN solutions, 0.1 M n-Bu4NBF4 electrolyte, glassy carbon working electrode, measured vs Ag/AgNO3 reference electrode. b This oxidation peak is coupled to a return peak Epc at -0.38 V. c This reduction peak is coupled to a return peak Epc at 0.28 V. d This value is for an irreversible reduction peak.

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Figure 8. Cyclic voltammograms of NiN2S2 (dashed line) and [NiN2S2]W(CO)4 (solid line) complexes in CH3CN solutions, 0.1 M n-Bu4NBF4 at a scan rate of 200 mV/s.

As shown in Figure 8d, the only redox event in the CV of CH3CN solutions of (Et4N)2[Ni(ema)] is a reversible oxidation at -0.30, which earlier was assigned by Holm and co-workers to NiII/III; the assignment was supported by EPR spectroscopy.35 In contrast, the CV of the tungsten derivative of Ni(ema)2-, [Ni(ema)W(CO)4]2-, also as its Et4N+ salt, reveals both an irreversible oxidation at -0.12 V and an irreversible reduction at -2.34 V under the same experimental conditions as used for study of Ni(ema)2-. Making the assumption that the complexation of the NiN2S22- by W(CO)4 shifts the NiII/I reduction potential at minimum by the same amount as in the neutral compounds, we estimate the NiII/I reduction in the free Ni(ema)2- to be -2.8 V or even more negative. As the shifts in NiII/I reduction potentials reflect the drain of electron density from the NiN2S2 coordination environment upon formation of the W(CO)4 adduct, one might expect a correlation of the differences in reduction potential with the ν(CO) IR data. That is, the greater shift in the value E1/2 for the NiII/I couple in the Ni(bme-Me2PDA) versus complex 3 as contrasted to analogous features in complex 1 would appear to indicate better

NiN2S2 Complexes as S-Donor Metallodithiolate Ligands

donor properties for the Ni(bme-Me2PDA) ligand over the Ni-1* ligand. Unfortunately, the infrared and force constant data are fairly indistinguishable within the series of neutral Ni-W compounds. It is not known whether the lack of correlation of ν(CO) and the NiII/I couple E1/2 values is due to an inherent insensitivity of the former to small differences in donor property, or whether there should be such a correlation at all. Stability of the (NiN2S2)W(CO)4 Complexes. The neutral complexes of this type have considerable thermal stability, even in solution, as evidenced by the VT NMR studies which demonstrated complex integrity at temperatures g100 °C. With the exception of the (Et4N)2[Ni(ema)W(CO)4], all of the NiN2S2W(CO)4 are, for practical purposes, air stable. As described above in the isolation of complex 7, thermal decomposition of complex 4 over a 1 week period in a sealed vial, where any liberated CO could be taken up by another molecule of 4, resulted in the pentacarbonyl species and conversion of the bidentate into a monodentate ligand. Subsequent studies of this hemilabile property with the deliberate addition of exogenous CO have shown that the pentacarbonyl complexes do not easily undergo complete CO displacement of the NiN2S2 ligand. Furthermore, preliminary studies find in all cases the ring closing, reformation of the tetracarbonyl, is in equilibrium with the opening/CO capture process (eq 1). The ring-opening characteristic of the Ni(µ-SR)2W core that produces an open site on the W(CO)4 moiety, yet persists as a nickelthiolato ligand, is an attractive feature for catalyst design. A related scenario is prominent in the computed mechanism for the ACS enzyme activity.8 A kinetic study of the ring-opening process in (NiN2S2)W(CO)4 complexes as they take up CO and convert to (NiN2S2)W(CO)5, as defined in eq 1, is the subject of a separate report.

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Figure 9. Structural overlay of (NiN2S2)W(CO)4 complexes in order of decreasing dihedral angle with hydrogen atoms omitted: purple (Ni-1*)W(CO)4, blue (Ni-1)W(CO)4, green (Ni-1′)W(CO)4, orange Ni(bmmpdmed)W(CO)4, yellow [Et4N]2[Ni(ema)W(CO)4], red [Ni(bme-Me2PDA)W(CO)4]. (a) Bond representations as cylinders using NiN2S2W(CO)4 only; (b) hydrocarbons added.

the (Ni-1)PdMeCl complex could represent the natural dihedral angle limit for NiN2S2 derivatives. We have noted that such a steric effect as defined for the NiN2S2 ligands may become a benefit for substrate orientation in palladium-promoted coupling reactions.21 Importantly, the VT NMR studies establish that this orientation is fixed, even to relatively high temperatures.

The electron-donating ability of the NiN2S2 ligands, related to each other and to other neutral donors to a first approximation through the ν(CO) stretching frequencies of W(CO)4 adducts, finds close analogies to N-donor ligands, such as piperidine or bipyridine; the ranking is as follows:

Summary and Comments

This detailed inspection of a series of NiN2S2 derivatives of W(CO)4 has identified a unique steric property of these metallodithiolate ligands to lie in the hinge angle imposed by the bridging thiolate sulfurs and their remaining quiescent lone pairs. There is no obvious correlation of the hinge angle with other metric data of the complexes such as the S-W-S bite angle or W-S distances. A detailed molecular mechanics analysis is necessary in order to assess all of the torsion angles within the polydentate ligand framework that influence the S-donor site and the dihedral or hinge angle. Nevertheless, from the crystallography data produced for this set of six complexes, we can conclude that substituents and orientations at the CR to S produce the principal steric deviation from the inherent dihedral angle of the NiN2S2/S2W(CO)2 planes as set by the S-lone pair/W-acceptor orbital interactions. The overlay of structures in Figure 9 shows an impressive range of the steric effect seen for the W(CO)4 series of complexes. The smallest of these is only 6° larger than the 101.3° dihedral angle observed for the (Ni-1)PdMeCl complex shown below.21 With no steric interactions possible in the two hinged square planes,

While the vibrational spectroscopy data do not distinguish between the neutral NiN2S2 ligands, the dianionic Ni(ema)2ligand significantly enriches the W(CO)4 moiety with electron density as contrasted to the neutral analogues. Interestingly, the average ν(CO) for [Ni(ema)W(CO)4]2- (1867 cm-1) is only 18 cm-1 lower than the average ν(CO) values of the neutral complexes (1885 cm-1). In contrast, the average ν(CO) value of a dianionic dithiolate, (S,S-C6H4)W(CO)42-, is 1850 cm-1 and that for a dithioether, (ButS(CH2)2SBut)W(CO)4, is 1920 cm-1, a 70 cm-1 difference.52,53 In summary, the electron density transferred to the W(CO)4 unit from the various bidentate S-donors is ranked as follows and denotes the buffer effect of (52) Dobson, G. R. Inorg. Chem. 1969, 8, 90-95. (53) Darensbourg, D. J.; Draper, J. D.; Frost, B. J.; Reibenspies, J. H. Inorg. Chem. 1999, 38, 4705-4714. J. AM. CHEM. SOC.

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Ni(II) on the anionic S-donor.

RS-R′-SR , Ni(SR)2 < Ni(SR)22- < -SR′SElectrochemical studies find significant differences in accessibility of the NiII/I redox couple in the NiN2S2 metallothiolate ligands upon coordination of the thiolate sulfur’s lone pairs to an exogenous metal. The differences in reduction potential appear to be more ligand dependent than are the ν(CO) and force constant values. Hence, while the IR spectroscopic analysis finds no differences in donor ability of the neutral NiN2S2 metallothiolate ligands, the NiII/I redox couples, specifically the difference between the E1/2 of the NiII/I couple in the free NiN2S2 ligand versus that bound to W(CO)4, suggest the order of electron withdrawal from the NiN2S2 ligands by W(CO)4 to be

Ni(bme-Me2PDA) > Ni(bmmp-dmed) > Ni-1′ > Ni-1 ≈ Ni-1* Interestingly, there is a significant, and as of now ill-defined, correlation of the hinge angles with the differences in ∆E1/2 of the NiII/I redox couples (∆E1/2 ) E1/2(NiII/I of NiN2S2W(CO)4) - E1/2(NiII/I of NiN2S2)), the understanding of which will require computational chemistry. Notably, the dianionic Ni(ema)2- complex, that which most faithfully models the Ni(Cys-Gly-Cys) moiety in the ACS enzyme, was found within our series to most prominently display the property of hemilability, releasing one S-W bond and opening up a site on the W(CO)4 moiety for the uptake of CO, producing a W(CO)5 complex in which the NiN2S22- ligand is monodentate. This observation is consistent with the in silico mechanism for the ACS enzyme active site in which the Ni(Cys-Gly-Cys)2- ligand switches into a monodentate form to accommodate addition of CO and switches back to re-form the resting state of the enzyme.8,9 Another interesting feature of the dianionic Ni(ema)2- complex ligand is how easily it fits into the overall spectroscopic and reactivity pattern of the neutral NiN2S2 ligands. This result is compatible with the work of Hegg et al., which demonstrated an array of electrophile reactivity

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with Ni(ema)2- that mirrored the S-based reactivity of neutral NiN2S2 complexes.19 Consistent with these similarities is the small change in ν(CO) stretching frequencies of the W(CO)4 derivatives of the dianionic as compared to the neutral nickeldithiolato complexes described above. Thus, in addition to moderation of charge by the nickel ion, it appears to buffer the donor ability. For the purposes of synthetic design and for analysis of donor properties, we have treated the NiN2S2 complexes as “innocent” ligands.54 This very useful assumption is arguably correct for the firm binding site for nickel in the tetradentate, square planar N2S2 coordination environment in combination with soft and low-valent metals, such as W0, Pd2+, and Ni(II/0). It provides a rationale for the assembly of the ACS active site, a rationale that can possibly be extended to other binuclear active sites in biology and presumably in organometallic applications. However, whether this innocence will be maintained in combination with all metals is not known. The full potential of these ligands should be addressed through systems designed to take advantage of their unique properties, some of which we have identified, and some of which await further studies. Acknowledgment. We gratefully acknowledge the financial support from the National Science Foundation (CHE 01-11629 and CHE 02-34860), Robert A. Welch Foundation, and the National Institutes of Health for a the Ruth L. Kirschstein-NRSA fellowship (to M.V.R., F31 GM073350-01). We appreciate a gift of the Ni(bmmp-dmed) from Dr. Craig A. Grapperhaus, University of Louisville. Supporting Information Available: Additional syntheses of (NiN2S2)W(CO)4 complexes; X-ray crystallographic files in CIF format for the structure determination of complexes 4-7. This material is available free of charge via the Internet at http://pubs.acs.org. JA055051G (54) The term “innocent” in this case refers to the stability of the NiII and RSredox levels throughout the reactions explored. However, the combination of a redox-active metal and thiolate sulfurs lends caution to this assumption. Ray, K.; Weyhermu¨ller, T.; Neese, F.; Wieghardt, K. Inorg. Chem. 2005, 44, 5345-5360.